Sinden represents the UK on the Artistic Advisory Board of the Colorado Festival of World Theatre, is a Founder member of the Piccadilly Dance Orchestra's Honorary Advisory Development Board and broadcast a regular live monthly slot 'UK Theatre News' on 'The Magazine' programme for BBC Radio Guernsey.

In August 1998 Sinden was producing a documentary at Vimy Ridge in Arras, northern France about the First World Warunderground mines (as mentioned in the Sebastian Faulks novel Birdsong) when Lt-Col Mike Watkins, the Head of Explosive Ordnance Disposal and the British Army's leading bomb disposal expert[18] who was in charge of the decommissioning of the huge bombs (including one of 6,500 lb) in the 20-mile subterranean complex, was killed in an accident underground.[19] Sinden said that Lt-Col Watkins was "A hero who had saved countless lives through his bomb disposal work. His work was often secretive and undercover and it would have put him in danger to publicise his action while he was alive, but now he is dead it does not matter. I admired him more than anyone I have ever met."[20]

In their review of the first 10 episodes of the series, the British Theatre Guide said "This film is as close as one can get to standing on the stage taking an ovation. This series is beautifully filmed and gets the balance exactly right between classy camera work, history, reminiscence and gossip."[26] The Daily Telegraph review stated that the "lovely documentary series is made by the director Marc Sinden. Its star, and – it transpires – the best documentary frontman of all time, is his actor-father: Sir Donald Sinden. Sir Donald has been let loose, offering anecdotes and memories apparently as they occur to him and the effect is enchanting beyond belief. It is also, at times, incredibly funny. It seems to me rather important that the series should be completed: this is popular history at its best."[27]

The first 10 episodes were broadcast from 3 August 2013 in the UK by the BSkyB digital satellite channel Sky Arts 2.[28]

Sinden is divorced from the film producer Jo Gilbert and has two children from that marriage: Hal Sinden (born 1980) who sings with his band Talanas[39] and Bridie Sinden (born 1990) who works at Sky News.[10] He is the son of actor Sir Donald Sinden (deceased) and his late wife, actress Diana Mahony[40] and the brother of the late actor Jeremy Sinden.[41]

In 1962, aged 8, Sinden was originally offered the lead title role in the film Sammy Going South by its director, Alexander Mackendrick, but his father turned the offer down on his son's behalf, saying that "only a handful of child actors ever make it as adult actors and if Marc wants to be an actor, he should wait until he is old enough to make the decision himself."[42]

Investigative journalist James Montague, writing in the July 2014 issue of Esquire magazine, claimed that Sinden spied for the British Government's Secret Intelligence Service (MI6) during the filming of Clash of Loyalties in Iraq, after being made "an offer he couldn’t refuse, appealing to his duty and his pride in Queen and Country." In the article Sinden admited that it was true.[46]